Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $19.86   6.511%   $21.1531   $21.15   $21.15   $21.20  
  Purchase #2 $50.44   3.420%   $52.165   $52.17   $52.15   $52.20  
  Purchase #3 $180.47   12.140%   $202.3791   $202.38   $202.40   $202.40  
  Purchase #4 $25.92   11.163%   $28.8134   $28.81   $28.80   $28.80  
  Purchase #5 $204.53   12.678%   $230.4603   $230.46   $230.45   $230.50  
  Purchase #6 $50.80   4.293%   $52.9808   $52.98   $53.00   $53.00  
  Purchase #7 $174.95   8.404%   $189.6528   $189.65   $189.65   $189.70  
  7 purchase totals = $706.97       $777.6045   $777.60   $777.60   $777.80